411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nc3/3...Nf6/4. e5/4...Nfd7
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Classical Defence/Steinitz Variation
Black retreats their Knight to d7, and now it is up to white to decide how to continue. White's most common response by far is 5. f4, gaining a significant space advantage whilst opening the e1-h4 diagonal to White's King. Black then typically responds with 5...c5, striking back in the center against White's strong pawn front.
Other uncommon moves here by white include Nf3, Nce2 and Be3.
Theory table
5. f4 c5 5. Nf3 5. Nce2
References
French Defence